1. Field of the Invention
This invention pertains to material handling, and more particularly to apparatus for feeding granular particles.
2. Description of the Prior Art
Various equipment has been developed to feed small amounts of particulate material from a storage container to a point of use. Thermal spray products such as flame spray and plasma spray equipment constitute an important application of particulate feeders. In such equipment, small amounts of metallic or ceramic powder are accurately metered from a hopper to a powder out port. From the powder out port, the powder is entrained in a gaseous stream for transportation to a thermal spray plasma or combustion gun. Examples of prior powder hoppers may be seen in U.S. Pat. Nos. 3,517,861; 3,909,068; 4,808,042; and Great Britain Patent Application 2,177,335A.
In the devices of the foregoing patents, a powder carrier wheel having accurately sized pockets or cavities rotates within a hopper. The wheel cavities receive definite volumes of powder from a large quantity in the hopper. Carrier wheel rotation carries the powder in the wheel cavities to a discharge location, where the powder is entrained in a gaseous stream for transportation to a spray gun. The rate of powder feeding is governed by the rotational speed of the carrier wheel and the size of the holes in the carrier wheel.
To assure that only the amount of powder that is containable within the wheel pockets or cavities is actually carried by the wheel to the discharge location, that is, that there is not a heap on the pocket or cavity, some prior hoppers employ leveling or smoothing devices. For example, the powder feeder of the U.S. Pat. No. 3,909,068 includes a wiper appended on a gasket for leveling the powder in the carrier wheel cavity. In the hopper of the U.S. Pat. No. 4,808,042, the carrier wheel cavities pass under a portion of the hopper wall, which scrapes away excess powder.
FIGS. 1 and 2 show an improved design of the powder hopper of the U.S. Pat. No. 3,517,861. The improvement consists of the addition of a seal 1 installed in a hopper housing 5 proximate a rotating powder carrier wheel 3. The seal 1 has a lip 6 with a planar bottom surface 10 that rubs on the top surface 8 of the wheel 3 as the wheel is rotated by a shaft 4. The seal lip 6 accurately levels the powder within a metering cavity 7 before the cavity and powder contained therein approach a powder out port 9. (For clarity the powder out port 9 is shown 90 degrees out of position.)
A characteristic of thermal spray powders is their extreme hardness and abrasiveness. Consequently, the powders tend to wear the hopper components that level the powder in the metering cavity. For instance, the gaskets of the U.S. Pat. No. 3,909,068 wear during use such that the housings that hold the gaskets must be periodically repositioned. With the seal 1 of FIGS. 1 and 2, some powder particles inevitably creep into voids between the lip bottom surface 10 and the carrier wheel top surface 8. The abrasiveness of the powder eventually wears the lip bottom surface 10 such that it does not properly prevent powder from passing under it and to the powder out port 9. In many cases the seal wears to such an extent that a gap is created between the lip bottom surface 10 and the carrier wheel top surface 8. As a result, the accuracy of the wheel metering system is degraded. Another detrimental effect of seal wear is that the powder gas can siphon powder from the hopper through the gap between the seal surface 10 and the powder carrier wheel 3 to the powder out port 9. Consequently, the powder feed rate is subject to considerable additional variation that is dependent on seal condition. In some applications, the service life of the seal 1 is between approximately 10 and 50 hours, depending on the particular powder being metered.
To assure accurate filling and discharge of the powder by a carrier wheel or similar metering device, it is known to utilize powder tampers. Powder tampers are especially useful to assist feeding low density or extremely fine powders that have a tendency to bridge or clog. U.S. Pat. Nos. 4,808,042 and 3,517,861 describe representative powder tampers. FIGS. 3 and 4 depict a tamper assembly that may be utilized with the hopper and seal design of FIGS. 1 and 2. Rotation of a cam wheel 11 causes spokes 13 extending radially from the cam wheel periphery to strike a lift cam 15. The lift cam 15 is joined to a leaf spring 17. In turn, the leaf spring 17 is captured between a washer 19 and a shoulder 21, which are retained by a thumb screw 22 on a post 23 that is connected to the shaft 4 of FIG. 1. The leaf spring has a central hole 24 for locating on the post 23. To the end of the leaf spring 17 is mounted a tamper bar 25 that is aligned with the carrier wheel cavities 7, FIG. 1. As the cam wheel 11 rotates, the spokes 13 strike the lift cam 15 to bend the leaf spring 17 in the region of the hole 24, such that the leaf spring deflects to the phantom position 17', thus raising the tamper bar 25. As a spoke 13 passes the lift cam, the lift cam is released, and the resilient nature of the leaf spring material snaps the leaf spring and tamper bar back to their undeflected positions of FIG. 4. Consequently, any powder under the tamper bar 25 is tamed into the wheel cavity 7.
The bending stress on the leaf spring 17 is heavily concentrated at the outer diameter of the washer 19. That stress concentration, the continuous flexing of the leaf spring, and the rigid construction of the washer 19, shoulder 21, knob 22, and post 23 cause the spring to wear undesirably quickly around the margins of the hole 24.
Thus, a need exists for improvements in powder hoppers.